1. Field of the Invention
The present invention relates generally to systems for receiving digital signals, and in particular, to systems for receiving layered modulation in digital signals.
2. Description of the Related Art
As various digital signal communication systems and services evolve, there is a burgeoning demand for increased data throughput and added services. However, it is more difficult to implement improvements in old systems and new services when it is necessary to replace existing legacy hardware, such as transmitters and receivers. New systems and services are advantaged when they can utilize existing legacy hardware. In the realm of wireless communications, this principle is further highlighted by the limited availability of electromagnetic spectrum. Thus, it is not possible (or at least not practical) to merely transmit enhanced or additional data at a new frequency.
The conventional method of increasing spectral capacity is to move to a higher-order modulation, such as from quadrature phase shift keying (QPSK) to eight phase shift keying (8PSK) or sixteen quadrature amplitude modulation (16QAM). Unfortunately, QPSK receivers cannot demodulate 8PSK or 16QAM signals. As a result, legacy customers with QPSK receivers must upgrade their receivers in order to continue to receive any signals transmitted with an 8PSK or 16QAM modulation.
Layered modulation enables systems and methods of transmitting signals to accommodate enhanced and increased data throughput without requiring additional frequency bands. Systems using layered modulation can provide enhanced and increased throughput signals for new receivers while remaining compatible with legacy receivers. Newer layered modulation techniques (such as detailed in U.S. patent application Ser. No. 09/844,401, filed Apr. 27, 2001, and entitled “LAYERED MODULATION FOR DIGITAL SIGNALS) also provide the unique advantage of allowing transmission signals to be upgraded from a source separate from the legacy transmitter. In other words, the layered signals can be asynchronous and/or non-coherent.
Related receiver systems for layered signals have also been described, such as those found in U.S. Pat. No. 4,039,961, which is incorporated by reference herein. However, such receiver systems are based on analog circuits, synchronized by a voltage control oscillator. In addition, such receiver systems are limited because they are designed to only receive coherent layered signals, i.e. signals that are synchronously produced.
Accommodating legacy receivers is also an important consideration when layered modulation is employed to enhance a preexisting system. Although proper design of the layered modulation signal can enable legacy receivers to receive legacy layers of the signal, the new signal layers will not be accessible by legacy receivers. In addition, it may not always be possible (or preferable) to accommodate the legacy receivers in designing the new layered modulation signal. In which case, the legacy receivers would be rendered incompatible with the new layered modulation signal.
There is a need for systems and methods for receiving and processing the layered modulation signals. There is also a need for systems and methods to enable legacy receivers to receive all layers of the layered signal. There is further a need for systems and methods which enable legacy receivers to be operable if the layered modulation signal is otherwise incompatible with the legacy receiver. The present invention meets these needs.